Modular organization of human leg withdrawal reflexes elicited by electrical stimulation of the foot sole.

Human withdrawal reflex receptive fields were determined for leg muscles by randomized, electrical stimulation at 16 different positions on the foot sole. Tibialis anterior, gastrocnemius medialis, peroneus longus, soleus, rectus femoris, and biceps femoris reflexes, and ankle joint angle changes were recorded from 14 subjects in sitting position. Tibialis anterior reflexes were evoked at the medial, distal foot and correlated well with ankle dorsal flexion. Gastrocnemius medialis reflexes were evoked on the heel and correlated with plantar flexion. Stimulation on the distal, medial sole resulted in inversion (correlated best with tibialis anterior activity), whereas stimulation of the distal, lateral sole evoked eversion. Biceps femoris reflexes were evoked on the entire sole followed by a small reflex in rectus femoris. A detailed withdrawal reflex organization, in which each lower leg muscle has its own receptive field, may explain the ankle joint responses. The thigh activity consisted primarily of flexor activation.     

Gradual enlargement of human withdrawal reflex receptive fields following repetitive painful stimulation

Dynamic changes in the topography of the human withdrawal reflex receptive fields (RRF) were assessed by repetitive painful stimuli in 15 healthy subjects. A train of five electrical stimuli was delivered at a frequency of 3 Hz (total train duration 1.33 s). The train was delivered in random order to 10 electrode sites on the sole of the foot. Reflexes were recorded from tibialis anterior, soleus, vastus lateralis, biceps femoris, and iliopsoas (IL). The RRF changes during the stimulus train were assessed during standing with even support on both legs and while seated. The degree of temporal summation was depending on stimulation site. At the most sensitive part of the RRF, a statistically significant increase in reflex size was seen after two stimuli while four stimuli were needed to observe reflex facilitation at less sensitive electrode sites. Hence, the region from which reflexes could be evoked using the same stimulus intensity became larger through the train, that is, the RRF was gradually expanding. Reflexes evoked by stimuli four and five were of the same size. No reflex facilitation was seen at other stimulus sites outside the RRF. In all muscles except in IL, the largest reflexes were evoked when the subjects were standing. In the ankle joint, the main withdrawal pattern consisted of plantar flexion and inversion when the subjects were standing while dorsi-flexion was prevalent in the sitting position. Up to 35 degrees of knee and hip flexion were evoked often leading to a lift of the foot from the floor during standing. In conclusion, a gradual expansion of the RRF was seen in all muscles during the stimulus train. Furthermore, the motor programme task controls the reflex sensitivity within the reflex receptive field and, hence, the sensitivity of the temporal summation mechanism.     

Modulation of the withdrawal reflex during hemiplegic gait: effect of stimulation site and gait phase.

OBJECTIVE: The objective of the study was to investigate the sensitivity of the nociceptive withdrawal reflex to stimulation of different locations on the sole of the foot during hemiplegic gait. METHODS: Reflexes were evoked by cutaneous electrical stimulation of 4 locations on the sole of the foot of 7 hemiplegic and 6 age-matched healthy persons. The stimuli were delivered at heel-contact, during foot-flat, at heel-off, and during mid-swing. Reflexes were recorded from muscles of the stimulated and the contralateral leg. Ankle, knee, and hip joints angles were recorded using goniometers. RESULTS: In the hemiplegic persons, the size of tibialis anterior reflexes, and the latency of soleus reflexes were site- and phase-modulated. In both groups, the tibialis anterior reflexes were significantly smaller with stimulation to the fifth metatarsophalangeal joint and the heel compared with the first metatarsophalangeal joint and the arch of the foot. The tibialis anterior reflexes evoked at heel-off and mid-swing were larger in hemiplegic persons than in healthy persons. Reflexes in the proximal and contralateral limb muscles were not site-modulated during hemiplegic gait. The kinematic response at the ankle joint was also different in the two groups during mid-swing. CONCLUSIONS: Hemiplegic and healthy middle-aged people presented different phase-modulation of the kinematic and muscle nociceptive reflex responses evoked by stimulation delivered on the sole of the foot. SIGNIFICANCE: The results have potential application in programs to rehabilitate hemiplegic gait.     

The action of plantar pressure on flexion reflex pathways in the isolated human spinal cord.

OBJECTIVE: To investigate the conditioning effects of plantar pressure on flexion reflex excitability in patients with motor complete spinal cord injury (SCI). METHODS: In five motor complete SCI subjects, the non-nociceptive flexion reflex was evoked via electrical stimulation of the right sural nerve and was recorded from the ipsilateral tibialis anterior muscle. Pressure ranging from 25 to 80kPa was applied to the metatarsal heads through an adjustable platform incorporated into a foot rest and a comparison of the reflex size made between control conditions and during pressure application. RESULTS: In all subjects, a significant depression of the long latency flexion reflex was observed when pressure was applied to the foot sole. The short latency flexion reflex appearing at latencies less than 100ms was absent in all patients. CONCLUSIONS: The results demonstrate that flexion reflex excitability in the isolated human spinal cord can be modulated by adequate activation of plantar mechanoreceptors. SIGNIFICANCE: Activation of plantar mechanoreceptors is a feature of normal standing and walking. Rehabilitation for standing and walking in SCI commonly uses body weight support based protocols. The strong inhibitory actions of plantar pressure on reflex pathways in the isolated human spinal cord suggest that sensory feedback from the foot sole may be an important factor in successful rehabilitation of standing and stepping in SCI patients.     

Effect of stimulation of the spinal cord on phasic and tonic reflexes in patients with central motor neuron damage

Spinal cord stimulation (SCS) is a method enabling the control of increased muscle tonus to be achieved in various spinal cord injuries. Polyelectromyographic (PEMG) methods were used for neurophysiological assessment of the degree of cord damage and persistent spinal reflexes as well as supramedullary influences. The analysed material comprised 40 PEMG records in 19 patients with spastic paraparesis or paraplegia after cord injury, cord tumour or multiple sclerosis. In 15 cases tentative epidural cord stimulation was done and 11 patients received implantation of a system for long-term stimulation. In most cases the epidural electrodes were implanted below the damaged segment, usually in the thoracic part of the cord. Before and after SCS beginning PEMG was done with a 16-channel Mingograph Siemens Elema with simultaneous recording of the responses from the symmetric muscles: quadriceps, semitendinous, adductor femoris, anterior tibialis and triceps surae. The effect of SCS was analysed on exteroceptive and proprioceptive reactions during testing of knee and ankle reflexes, and on the response of the muscles to vibration. In most patients a reduction was observed of the intensity of tendon reflexes, particularly the spread of the reflex to the contralateral extremity was no longer seen. The vibration reflex had a tonic character persisting in 48% of the studied muscles, even in patients with clinically complete transsection of the cord. The change of the character of monosynaptic reflexes and the presence of the vibration reflex suggest that SCS modifies the proprioceptive segmental spinal reactions.     

Human flexor reflexes

One type of flexor reflex, that recorded from the tibialis anterior muscle in response to electrical stimulation of the sole of the foot, was studied in normal subjects and patients with several neurological disorders. Normally this reflex consists of two components, the second of which is related to the actual withdrawal. The first component, normally of lower threshold, is difficult to evoke in patients with chronic spinal cord or discrete cerebral lesions, whereas it has an unusually low threshold and is very clearly seen in those with Parkinson's disease. In patients with spinal cord disease, the exaggerated flexor reflexes are seen at long latencies after relatively small stimuli. During the early phase of recovery from spinal transection, both components may be seen and are, therefore, spinal in origin. Studies of patients with the sensory neuropathy of Friedreich's ataxia suggest that the afferent fibres responsible for these flexor reflexes are the small myelinated fibres. Recovery curves demonstrate very long-lasting changes in flexor reflex excitability in normal subjects and patients with `spasticity' from spinal lesions. This differs in patients with `spasticity' from lesions rostral to the brain-stem. Examples in man of such physiological phenomena as reciprocal inhibition, local sign, habituation, temporal and spatial summation are discussed.     

Nociceptive withdrawal reflexes evoked by uniform-temperature laser heat stimulation of large skin areas in humans

Nociceptive withdrawal reflexes (NWR) were evoked by brief (200 ms) painful CO(2) laser stimulation at five intensities (1.2, 1.4, 1.6, 1.8, and 2.0 x pain threshold) applied to nine sites (2 cm(2)) separated by 1.7 cm on the dorsal side of the foot and anterior part of the lower leg of 14 healthy volunteers. The purpose of the study was to investigate the characteristics of NWRs evoked by a natural stimulation modality. The reflexes were measured as the electromyographic response from the iliopsoas (ILI), quadriceps vastus lateralis (QVL), biceps femoris (BF), tibialis anterior (TA), and soleus (SOL) muscles. Stimulus-response relationships between heat intensity and the reflex magnitude and correlation between perceived pain intensity and reflex magnitude were observed in the ILI, QVL, BF, and TA but not the SOL. No significant differences in reflex magnitude were found between the stimulation sites. NWRs were evoked more often in flexor muscles than extensor muscles, indicating a non-site-specific reflex organization. The paper presents a new method to evoke NWRs by uniform-temperature laser heat stimulation of large skin areas in humans. These heat evoked reflexes had a stimulus-response relationship.     

Neuronal function in chronic spinal cord injury: divergence between locomotor and flexion- and H-reflex activity.

OBJECTIVE: Knowledge about the long-term course of spinal neuronal function after spinal cord injury (SCI) is important if regeneration therapies become available in the future. The objective of this study was to examine the behavior of locomotor EMG activity and of spinal reflexes in patients with chronic motor-complete SCI. METHODS: EMG activity from rectus femoris (RF), biceps femoris (BF), medial gastrocnemius (GM) and tibialis anterior (TA) of both sides was investigated during locomotor movements assisted by a robotic device in 10 chronic (>1 year after accident) complete SCI and 5 healthy subjects. H-reflexes (recorded from GM) were induced during the onset and flexion reflexes (recorded from BF and TA) at the end of the stance phase. RESULTS: Only in the chronic SCI subjects an exhaustion of EMG activity--i.e. a decrease in amplitude--occurred within a few minutes in all leg muscles. The EMG exhaustion was not associated with a change in the H- or flexion reflex amplitude during a walking session. CONCLUSIONS: Exhaustion of neuronal function in chronic complete SCI might be restricted to unused motor tasks, i.e. locomotion. The fact that H- and flexion reflexes show a normal behavior might be due to the fact that they still become activated. SIGNIFICANCE: Training/pharmalogical approaches may be required to maintain neuronal function of unused tasks as a basis for future successful regeneration therapies.     

Reflex receptive fields for human withdrawal reflexes elicited by non-painful and painful electrical stimulation of the foot sole.

OBJECTIVES: Human withdrawal reflex receptive fields (RRFs) were assessed for 4 different electrical stimulus intensities, ranging from below the pain threshold (PTh) to up to two times the PTh intensity (0.8x, 1.2x, 1.6x, and 2.0xPTh). METHODS: Thirteen subjects participated, and the reflexes were recorded in a sitting position. The stimuli were delivered in random order to 12 positions distributed over the foot sole. Tibialis anterior (TA), gastrocnemius medialis (GM), vastus lateralis (VL), and biceps femoris (BF) reflexes were recorded. Further, knee and ankle joint angle changes were recorded. RESULTS: The strongest reflexes were seen in the TA compared with the other 3 muscles. Dorsi-flexion dominated distal to the talocrural joint corresponding to the TA receptive field area. An expansion of the RRF for the TA and GM was seen when increasing the stimulus intensity from 0.8xPTh to 1.2xPTh and from 1.2xPTh to 1.6xPTh, indicating a gradually increasing reflex threshold towards the border, where TA contraction is inappropriate in a withdrawal reaction. For the BF and VL, the borders of the RRF areas were not detected. By integrating the reflex size within the RRF (i.e. the reflex volume), gradually increasing reflexes for increasing stimulus intensity were seen in all 4 muscles tested, most clearly in the TA and GM. The subjective pain intensity correlated to the reflex volume for the TA, GM, and BF. CONCLUSIONS: In conclusion, the highest reflex sensitivity was seen in the centre of the RRF, while the stimulus intensity needed for eliciting a reflex increased towards the receptive field border. Within the RRF, stronger reflexes were evoked for increasing stimulus intensity. The limit in the size of the receptive field size for the TA and GM supports a modular withdrawal reflex organisation.     

A flexible electrode array for determining regions of motor function activated by epidural spinal cord stimulation in rats with spinal cord injury

Epidural stimulation of the spinal cord is a promising technique for the recovery of motor function after spinal cord injury.The key challenges within the reconstruction of motor function for paralyzed limbs are the precise control of sites and parameters of stimulation.To activate lower-limb muscles precisely by epidural spinal cord stimulation,we proposed a high-density,flexible electrode array.We determined the regions of motor function that were activated upon epidural stimulation of the spinal cord in a rat model with complete spinal cord,which was established by a transection method.For evaluating the effect of stimulation,the evoked potentials were recorded from bilateral lowerlimb muscles,including the vastus lateralis,semitendinosus,tibialis anterior,and medial gastrocnemius.To determine the appropriate stimulation sites and parameters of the lower muscles,the stimulation characteristics were studied within the regions in which motor function was activated upon spinal cord stimulation.In the vastus lateralis and medial gastrocnemius,these regions were symmetrically located at the lateral site of L1 and the medial site of L2 vertebrae segment,respectively.The tibialis anterior and semitendinosus only responded to stimulation simultaneously with other muscles.The minimum and maximum stimulation threshold currents of the vastus lateralis were higher than those of the medial gastrocnemius.Our results demonstrate the ability to identify specific stimulation sites of lower muscles using a high-density and flexible array.They also provide a reference for selecting the appropriate conditions for implantable stimulation for animal models of spinal cord injury.This study was approved by the Animal Research Committee of Southeast University,China(approval No.20190720001) on July 20,2019.     

Spatial facilitation of motor evoked responses in monitoring during spinal surgery.

During spinal cord monitoring, motor responses in the tibialis anterior muscles were recorded on transcranial electrical stimulation of the motor cortex. In order to facilitate the responses, the cortical stimulus was preceded by a train of stimuli to the foot sole within the receptive field of the withdrawal reflex of the tibialis anterior muscle. This cutaneous input provides a spatial facilitation of the cortically elicited response. When the stimulus interval was 50-100 ms, large and reliable responses were seen in most cases.     

Central sensitization in spinal cord injured humans assessed by reflex receptive fields

ObjectiveTo investigate the effects of central sensitization, elicited by intramuscular injection of capsaicin, by comparing the reflex receptive fields (RRF) of spinally-intact volunteers and spinal cord injured volunteers that present presensitized spinal nociceptive mechanisms.MethodsFifteen volunteers with complete spinal cord injury (SCI) and fourteen non-injured (NI) volunteers participated in the experiment. Repeated electrical stimulation was applied on eight sites on the foot sole to elicit the nociceptive withdrawal reflex (NWR). RRF were assessed before, 1 min after and 60 min after an intramuscular injection of capsaicin in the foot sole in order to induce central sensitization.ResultsBoth groups presented RRF expansion and lowered NWR thresholds immediately after capsaicin injection, reflected by the enlargement of RRF sensitivity areas and RRF probability areas. Moreover, the topography of the RRF sensitivity and probability areas were significantly different in SCI volunteers compared to NI volunteers in terms of size and shape.ConclusionsSCI volunteers can develop central sensitization, despite adaptive/maladaptive changes in synaptic plasticity and lack of supraspinal control.SignificanceProtective plastic mechanisms may still be functional in SCI volunteers.     

Modular organization of excitatory and inhibitory reflex receptive fields elicited by electrical stimulation of the foot sole in man.

OBJECTIVES: The present study aimed to investigate how the inhibitory and excitatory reflex components of the human (polysynaptic) withdrawal reflex are organized depending on the stimulation site. The reflexes were elicited during a voluntary pre-contraction (between 10 and 20% of maximum voluntary contraction) of two antagonistic muscles. METHODS: Inhibitory and excitatory reflex receptive fields to tibialis anterior (TA) and soleus (SO) were mapped in 14 healthy subjects using randomized electrical stimulation at 16 sites of the foot sole. Low, non-painful (3x perception threshold), and high, painful (1.5x pain threshold), stimulus intensities were used. RESULTS: The inhibitory reflex receptive fields were organized in a highly functional manner supporting the action of the excitatory reflex. Together the two reflexes result in an optimal withdrawal from the stimulus. Low stimulation intensity was found sufficient to elicit the inhibitory reflex. High stimulation intensity caused a reversal of the inhibition to excitation in tibialis anterior. In soleus the inhibition was facilitated for stronger intensities. CONCLUSION: In conclusion, findings in animals of a modular organization of inhibitory reflexes are reproduced in humans.     

Parallel facilitatory reflex pathways from the foot and hip to flexors and extensors in the injured human spinal cord

Spinal integration of sensory signals associated with hip position, muscle loading, and cutaneous sensation of the foot contributes to movement regulation. The exact interactive effects of these sensory signals under controlled dynamic conditions are unknown. The purpose of the present study was to establish the effects of combined plantar cutaneous afferent excitation and hip movement on the Hoffmann (H) and flexion reflexes in people with a spinal cord injury (SCI). The flexion and H-reflexes were elicited through stimulation of the right sural (at non-nociceptive levels) and posterior tibial nerves respectively. Reflex responses were recorded from the ipsilateral tibialis anterior (TA) (flexion reflex) and soleus (H-reflex) muscles. The plantar cutaneous afferents were stimulated at three times the perceptual threshold (200 Hz, 24-ms pulse train) at conditioning-test intervals that ranged from 3 to 90 ms. Sinusoidal movements were imposed to the right hip joint at 0.2 Hz with subjects supine. Control and conditioned reflexes were recorded as the hip moved in flexion and extension. Leg muscle activity and sagittal-plane joint torques were recorded. We found that excitation of plantar cutaneous afferents facilitated the soleus H-reflex and the long latency flexion reflex during hip extension. In contrast, the short latency flexion reflex was depressed by plantar cutaneous stimulation during hip flexion. Oscillatory joint forces were present during the transition phase of the hip movement from flexion to extension when stimuli were delivered during hip flexion. Hip-mediated input interacts with feedback from the foot sole to facilitate extensor and flexor reflex activity during the extension phase of movement. The interactive effects of these sensory signals may be a feature of impaired gait, but when they are appropriately excited, they may contribute to locomotion recovery in these patients.     

Organization of stabilizing reflex responses in tibialis anterior muscles following ankle flexion perturbations of standing man

Reflex activity in human ankle muscles in response to 36 deg./s dorsi-flexion rotations of the feet was investigated in subjects standing upright and when leaning back so as to preactivate ankle flexor muscles. Short latency stretch reflex activity in soleus and inhibition in tibialis anterior muscles occured at 50 ms from ankle rotation onset. Two prominent bursts of tibialis activity followed at 83 and 131 ms, and preceded large stabilizing ankle torques. Head movements commenced 20 ms after foot rotations and acquired accelerations exceeding 100 deg./s² within 60 ms. It is suggested that the tibialis anterior activity is either a vestibulospinal reflex resulting rom the head movement, or a strethc reflex only present standing, since this activity was not observed when seated subjects received identical foot rotations.     

Differential Effects of a Distant Noxious Stimulus on Hindlimb Nociceptive Withdrawal Reflexes in the Rat

Recent studies indicate that the nociceptive withdrawal reflexes to individual muscles are evoked by separate reflex pathways. The present study examines whether nociceptive withdrawal reflexes to different muscles are subject to differential supraspinal control in rats. A distant noxious stimulus was used to activate a bulbospinal system which selectively inhibits 'multireceptive' neurons (i.e. neurons receiving excitatory tactile and nociceptive inputs) in the dorsal horn of the spinal cord. Withdrawal reflexes, recorded with electromyographic techniques in single hindlimb muscles, were evoked by standardized noxious pinch. Thirty-seven rats, anaesthetized with halothane and nitrous oxide, were used. Whereas withdrawal reflexes to the extensor digitorum longus and brevis, tibialis anterior and biceps posterior muscles were strongly inhibited, reflexes to interossei muscles were potentiated during noxious pinch of the nose. Reflexes to peronei muscles were not significantly changed. The effects on the reflexes usually had an onset latency of <0.5 s and outlasted the conditioning stimulation by up to 2 s. The monosynaptic la reflex to the deep peroneal nerve, innervating dorsiflexors of the digits and ankle, was not significantly changed during noxious pinch of the nose. Hence, the inhibitory effects on the hindlimb withdrawal reflexes induced by the conditioning stimulation were presumably exerted on reflex interneurons. It is concluded that nociceptive withdrawal reflexes to different hindlimb muscles are differentially controlled by descending pathways activated by a distant noxious stimulus. The results support our previous conclusion that there are separate nociceptive withdrawal reflex pathways to different hindlimb muscles.     

Cutaneous reflexes from the foot during gait in hereditary spastic paraparesis.

OBJECTIVE: It is known that P2 cutaneous reflexes from the foot show phase-dependent modulation during gait. The role of the motor cortex and the cortico-spinal tract in these reflexes and their modulation is unknown. Patients with hereditary spastic paraparesis (HSP) have a lesion in the cortico-spinal tract and may show deficits in P2 reflexes and/or their modulation. METHODS: Reflex responses of tibialis anterior and biceps femoris after sural nerve stimulation in 10 HSP-patients were compared with those in 10 healthy subjects. The reflexes were studied at two different moments in the step cycle during walking on a treadmill. RESULTS: Both patients and controls showed a phase-dependent modulation of P2 responses. For the individual muscles, no significant difference in reflex activity was observed between HSP-patients and the controls. However, when all muscles were taken together, the reflex activity for the controls was significantly higher than for the patients. CONCLUSIONS: The results of this study suggest that the cortico-spinal tract is involved in the regulation of the amplitude of the P2 responses and their phase-dependent modulation.     

Tibialis Anterior and Soleus Withdrawal Reflexes Elicited by Electrical Stimulation of the Sole of the Foot during Gait

The aim of the present study was to investigate the modulation and functional importance of nociceptive withdrawal reflexes elicited from the sole of the foot and recorded from the soleus (SOL) and tibialis anterior (TA) muscles during gait. Cutaneous electrical stimulation delivered at four locations of the sole of the foot was used to elicit the withdrawal reflex. Reflexes were recorded from eight healthy subjects during treadmill walking. The reflexes were elicited at heel‐contact, during foot‐flat, at heel‐off, and during mid‐swing. The reflexes evoked in TA were largest when the arch of the foot was stimulated, and smallest following stimulation of the heel (significant difference during stance, p ≤ 0.002). The largest soleus responses were elicited when the arch of the foot was stimulated (significant difference compared with the fifth metatarsophalangeal joint, stimulation after heel‐contact, p < 0.05). The TA reflex, expressed as a proportion of the electromyogram during unperturbed gait, was smallest during swing (p < 0.05, compared with stance) whereas the SOL reflex was maximal during swing (p < 0.05, compared with stance). The results suggest that the modulation of the reflex promotes an appropriate withdrawal while preserving balance and continuity of motion. These results may have applications in assisting gait of hemiplegics.     

Changes in spinal reflex excitability in brain-dead humans

The excitability of proprio- and exteroceptive spinal reflexes was monitored electrophysiologically and clinically during the occurrence of brain death (BD) in 8 patients. After a period of total reflex unresponsiveness, the soleus H reflex attained a steady-state excitability level in 2-6 h. The recovery cycle of this response regained its normal shape at 10-20 h. The threshold of the cutaneous reflex evoked in the biceps femoris by electrical stimulation of the sural nerve had become normal in 4-13 h, although the response displayed an abnormal multi-component pattern. Digital responses to mechanical stimulation of the foot sole were evident after 6-8 h. Knee and ankle jerks were never evoked during the time of monitoring. The time-courses of the changes in excitability were not directly correlated with the fall in the blood pressure which may occur during BD. It is concluded that the human spinal cord reacts to BD with a spinal shock, characterized by sequential recovery of reflex transmission. The overall timing of this process appears to be much shorter than that previously described for the spinal shock following traumatic transection of the cord, but the latter was never studied in the earliest phases.     

Flexor reflex decreases during sympathetic stimulation in chronic human spinal cord injury

A better understanding of autonomic influence on motor reflex pathways in spinal cord injury is important to the clinical management of autonomic dysreflexia and spasticity in spinal cord injured patients. The purpose of this study was to examine the modulation of flexor reflex windup during episodes of induced sympathetic activity in chronic human spinal cord injury (SCI). We simultaneously measured peripheral vascular conductance and the windup of the flexor reflex in response to conditioning stimuli of electrocutaneous stimulation to the opposite leg and bladder percussion. Flexor reflexes were quantified using torque measurements of the response to a noxious electrical stimulus applied to the skin of the medial arch of the foot. Both bladder percussion and skin conditioning stimuli produced a reduction (43–67%) in the ankle and hip flexor torques (p < 0.05) of the flexor reflex. This reduction was accompanied by a simultaneous reduction in vascular conductance, measured using venous plethysmography, with a time course that matched the flexor reflex depression. While there was an overall attenuation of the flexor reflex, windup of the flexor reflex to repeated stimuli was maintained during periods of increased sympathetic activity. This paradoxical depression of flexor reflexes and minimal effect on windup is consistent with inhibition of afferent feedback within the superficial dorsal horn. The results of this study bring attention to the possible interaction of motor and sympathetic reflexes in SCI above and below the T5 spinal level, and have implications for clinicians in spasticity management and for researchers investigating motor reflexes post SCI.